1. Technical Field
The present invention generally relates to material handling conveyors and, more particularly, to a transfer assembly for a chain driven conveyor system.
2. Discussion
A wide variety of conveyor systems are available in the art for transporting articles through a manufacturing facility. For example, power and free (PF) chain conveyor systems which generally include trolleys coupled to move a workpiece. The trolleys are commonly freely movable within a support track which is positioned to place the trolleys in operative engagement with a drive chain having pusher dogs. The dogs drivably engage the trolleys to move the trolleys through the manufacturing area.
In many PF conveyor systems the workpiece and associated trolleys are transferred between separate drive chains so that one of the chains can move the workpiece through a first work area and the second chain can move the workpiece through a second work area. A chain-to-chain transfer assembly is commonly provided in the PF conveyor system to transfer the trolleys between driving engagement with the first chain (the wipe-off chain) and the second chain (the wipe-on chain). More particularly, during the transfer operation, a pusher dog fixed to the wipe-off chain brings the trolley into a transfer zone where the wipe-off and wipe-on pusher dogs could simultaneously engage the trolley. To prevent simultaneous engagement, a mechanical cam is positioned to move the retractable dog of the trolley to a height where the wipe-off chain, but not the wipe-on chain, is engageable with the trolley. The wipe-off chain is therefore able to push the trolley to a point in the transfer zone where the trolley drops off the cam bar and into its engaging position where the wipe-on dog can move the trolley away from the transfer zone.
A representative prior art transfer operation is illustrated in FIGS. 1-5 and includes a wipe-off drive chain 10 that drives a trolley 14 into the transfer zone and a wipe-on drive chain 12 that drives the trolley out of the transfer zone. A cam bar 16 is positioned in the transfer zone between the wipe-off and wipe-on drive chains and is configured to operatively engage a retractable dog 18 on the trolley 14 (FIGS. 3 and 4) to transfer the trolley from driving engagement with the wipe-off drive chain 10 to driving engagement with the wipe-on drive chain 12.
More particularly, the wipe-off drive chain 10 drives the retractable trolley dog 18 onto the cam bar which is configured to raise the trolley dog from a lowered position to a raised position (shown in FIG. 4 and in shadow in FIG. 5). The wipe-off track then curves away from the cam bar such that the wipe-off pusher dog 20 (FIG. 4) is moved out of engagement with the trolley dog 18. If the trolley 14 has sufficient momentum, the trolley dog passes by the forward end 24 (FIGS. 1 and 2) of the cam bar, drops into its lowered engaging position, and is drivably engaged by the next wipe-on pusher dog 22. When the retractable dog 18 is in its raised position, the pusher dog 22 of the wipe-on drive chain 12 is positioned below and operably disengaged from the drive surface 25 of the retractable dog 18 (FIG. 4). The cam bar 16 maintains the retractable dog in this raised position until the rear surface of the retractable dog slides off the cam bar thereby permitting the retractable dog to return to its engaging position. The wipe-on chain dog 22 is then operably engageable with the lowered retractable dog 18 so as to push the carrier trolley out of the transfer zone.
The smooth operation of the PF conveyor system depends upon the efficient transfer of trolleys from the wipe-off drive chain to the wipe-on drive chain. Even a small percentage of missed transfers cause manufacturing inefficiencies. Missed transfers are most common in the prior art design when the wipe-off chain is moving slower than the wipe-on chain 12. With a slow moving wipe-off chain and correspondingly slow moving trolley, the trolley may have insufficient momentum to overcome the sliding friction between the riding surface 26 of the trolley and the cam bar. The trolley then occasionally gets caught up on the cam bar which maintains the retractable dog 18 in its retracted position and therefore not drivably engageable by the pusher dog 22 on the wipe-on drive chain 12. The trolley 14 then sits on the cam bar and obstructs the movement of the next trolley causing a blockage of trolley transfer.
A representative trolley, such as the Dog Magic(copyright) trolley manufactured by Jervis B. Webb Company of Farmington Hills, Mich., the assignee of the present invention, is illustrated in FIGS. 3-5 to include an anti-backup tail 28 located rearward of the riding surface 26. The riding surface 26 includes a lead end 30 and a trailing end 32 while the anti-backup tail 28 includes a rear tip 34 spaced from the trailing end 32 as indicated by reference numeral 36. When the trolley has insufficient forward momentum, the trolley 14 may come to rest with the tail 28 sitting on the cam bar thereby prohibiting the retractable dog 18 from fully returning to its engaging position and preventing the wipe-on pusher dog 22 from operatively engaging the dog 18.
In an attempt to decrease the frequency of the trolley getting caught up on the cam bar, the prior art has used various auxiliary pushing mechanisms (e.g., such as air cylinders that impact the rear of the trolley 14) to increase the forward momentum of the trolley. While these techniques have provided some increase in efficiency, 100% transfer, particularly during slow wipe-off and fast wipe-on speeds, have not been achieved.
In view of the above, a need exists for a cam bar design that more efficiently transfers trolleys from driving engagement with the wipe-off chain to driving engagement with the wipe-on chain. The improved cam bar design addresses both of the above-identified sources of trolley interference: the riding surface of the trolley getting caught up on the cam bar and the anti-backup tail remaining on the cam bar after the retractable trolley dog clears the downstream end of the cam bar. Accordingly, the present invention includes an improved cam bar design having a primary cam bar and a secondary cam bar that is longitudinally separated and axially offset from the primary cam bar. The many advantages of this arrangement include reducing the length of the dead zone, i.e., the zone where the wipe-off and wipe-on chains are each operably disengaged from the trolley and the trolleys momentum is relied upon to clear the cam bars, and arranging the offset of the split cam bars to permit the retractable dog to drop fully into its engaging position even if the trolley stops when the retractable dog is at the downstream end of the secondary cam bar. Under this condition, the anti-backup tail fully clears the first cam bar at approximately the same time the riding surface of the retractable dog drops off the secondary cam bar. This design virtually eliminates missed take-away chain dogs and reduces the possibility of drive torque outs due to trolleys jamming into pusher dogs.
Further scope of applicability of the present invention will become apparent from the following detailed description, claims, and drawings. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.